1. Field of the Invention
The present invention relates to an interference objective lens unit and a light-interference measuring apparatus using thereof.
2. Description of the Related Art
Conventionally, there is known a light-interference measuring apparatus such as a three-dimensional shape measuring apparatus. The three-dimensional shape measuring apparatus precisely measures the three-dimensional shape of a measuring object, for example, by using the luminance information of interference fringes generated by the interference of lights. A technique using a broad band light (white light and the like) as a light source is widely known in this light-interference measuring apparatus (see, for example, Japanese Patent Application Laid-Open Publication No. 2003-148921).
When the broadband light is used as the light source, the peaks of luminance of the interference fringes of the respective wavelengths overlap one another and the luminance of the overlapped interference fringes becomes larger at the focused position. However, the more distant from the focused position, the larger the peak luminance positions of the interference fringes of the wavelengths shift from each other and the amplitude of the luminance of the superposed interference fringes be smaller gradually.
Therefore, the light-interference measuring apparatus can consequently measure, for example, the three-dimensional shape of a measuring object by detecting the position of peak luminance at each position in a visual field.
The interference objective lenses to be used for such light-interference measuring apparatus mainly include Michelson type lenses and Mirau type lenses, which are used according to the magnification ratios of the interference objective lenses. In general, the Mirau type lenses are used for the interference objective lenses of high magnification ratios, and the Michelson type lenses are used for the interference objective lenses of low magnification ratios.
FIG. 7 is a schematic view showing the basic configuration of the Mirau type interference objective lens. FIG. 8 is a schematic view showing the basic configuration of the Michelson type interference objective lens.
As shown in FIGS. 7 and 8, an optical path of light emitted from the interference objective lens is branched by a beam splitter such as a beam splitter plate 133 or a beam splitter cube 183 into a reference optical path (denoted by the broken line in the drawing) including a reference mirror 132a, 182a therein and a measuring optical path (denoted by the solid line in the drawing) including a measuring object arranged therein. Thereafter, reflected light from the reference mirror (reference light) and reflected light from the measuring object (object light) are superposed by the beam splitter.
When the difference of the optical path lengths between the reference optical path and the measuring optical path is adjusted to be zero by the position of the beam splitter, the reference light and the object light have same phase with each other. Thus, the interference light wave is reinforced.
Therefore, when using an interference objective lens unit, the position of the beam splitter is set so that the reference light and the object light have same phase with each other as shown in FIG. 9.
At the position of the beam splitter where the reference light and the object light have same phase with each other, that is, where the focal plane of the interference objective lens and the reflecting plane of the reference mirror 132a are conjugated (see FIG. 10), interference fringes can be seen in clear contrast.
However, when environmental temperature of an interference objective lens changes after setting a position of the beam splitter, space between optical elements, curvature and refractive index of the objective lens and the like change. Thus, the focal length of the objective lens changes and the focal plane of the interference objective lens and the reflecting plane of the reference mirror 132a become unconjugated (see FIG. 11). As a result, phase difference between the reference light and the object light arises and the brightness of the interference fringes decreases.
In addition, the setting of a position of optical elements in the interference system to keep the brightness of the interference fringes, as environmental temperature changes is cumbersome.